Advice on making a fake Haunting Story unique.

I am planning on making a mystery novel soon focusing on a high-school aged detective ala Nancy Drew, except in the POV of his journal as he is investigating the case. So far, I have the setting of the story as an old Southern plantation where the recent owner, an elderly Southern gentleman and Bourbon Baron Buford Ellis meets his untimely end due to a brick falling from the bell tower of the chapel of the house and striking him on the top of the head, killing him instantly. At first, it looks like an accident, but a witness comes forward and states that he saw the brick fly out of the bell tower, as if tossed and strike Buford but claimed to see no one in the tower due to the roof, which casts much of the towers inside in shadows. This statement made the police suspect murder. But, when they went up to investigate the tower, they discover that the entrance locked from the inside and that there was no sign of everyone ever being in the tower.

This statement leads to an old rumor boiling back up that a violent and very angry poltergeist haunts the house and that the house and the land around it are cursed due to its sordid past. Throughout the story, the MC encounters increasingly dangerous paranormal phenomena as he investigates the murder. Eventually, it is revealed that there is a rumor that there is Confederate Gold hidden somewhere in the house and that the culprit is faking the haunting to scare everyone away so they can search for the treasure itself.

I know these types of stories have been done many times before. So, may i have some advice on making this story unique from all the other one out there?

Well, I don't know about anybody else, but I like this story premise a lot. It already sounds fairly unique. If you set the scene in that place, give us all the details we need to envision the place (which is exotic enough, if the reader isn't familiar with old Southern plantations) and the people, the story will work a treat.

A story like that requires detail and atmosphere. It's almost as if the murder (s) become secondary to making that setting and those people come alive for us.

I'm not a big reader of mysteries. However, as a child growing up in the USA in the 1950s and 60s, I was fascinated by Agatha Christie mysteries. In fact I read all of them, and many of them several times. The 'several times' is the clue here. I didn't read them because I was dying to find out whodunnit. In fact, I often didn't much care whodunnit. (It was a hallmark of Christie's style that ANYBODY could have dunnit.) Instead, I read Agatha Christie for the sense of local colour that her mysteries produced. Those little calendar-perfect villages with the dark human stories of the people who lived there—I ate that stuff up!

I think local colour and a sense of time and place can rescue even a fairly predictible plot. Yours sounds better than that. But work hard on creating that sense of time and place. And make your amateur detective into a real character. Surround him with other 'real characters.' And let us feel that Southern heat, that faded grandeur, that ever-simmering folk memory of 'the war.' It'll be a winner.